1. Field of the Invention
The present invention relates to a camera, and particularly to a camera with a zoom lens comprising a focusing means for correcting the deviation of a focal position, which is caused by a variation in a lens.
2. Description of the Related Art
In such a focussing means of a conventional camera, a zoom motor is rotated in a predetermined direction and then stopped for removing backlash which causes a deviation of a focal position as a zoom lens system is moved, as disclosed in U.S. Pat. No. 4,936,664. In another known focusing means, a moving amount of an autofocus lens at a typical zoom position has been previously stored in an ROM so that a shift amount from the value stored in the ROM at each zoom position is stored in a EE-PROM and compensated for by interpolation, as disclosed in U.S. Pat. No. 4,914,464.
However, the means disclosed in U.S. Pat. No. 4,914,464 has a feel of discomfort because the zoom lens system is moved in opposite directions when a zooming operation is made and when the finger pressure is released from a zoom button. The means disclosed in U.S. Pat. No. 4,914,464 has the disadvantage that consideration is given to correction only in the case where a zoom system is moved in one direction.
On the other hand, the amount a general zoom lens is moved for focussing optically depends upon a zoom position when the output of an AF (autofocus) sensor indicates close range, as shown in FIG. 22. Namely, the moving amount of the lens in a telescopic T (referred to as "tele" hereinafter) state is greater than that in a wide angle W (referred to as "wide" hereinafter) state. The difference between the moving amounts in both states is small at infinite range, and gradually increases as the range becomes closer. The focal distances in the W to T states are thus divided so that the extension pulse of the lens is selected according to the output of the AF sensor at a divided focal distance.
On the other hand, for example, a two-group zoom lens optical system uses, for moving the zoom lens system, a cam driving mechanism generally comprising grooved cams and longitudinal pins which are respectively engaged in the grooved cams. A driving grooved cam for moving a first lens group comprises a lead cam, and a driving grooved cam for moving a second lens groove comprises a nonlinear cam. The distance between the first and second lens groups is large in the W (wide) state and is small in the T (tele) state. This is because the cam diagram is formed so that the focusing amounts fc in the W to T states are the same at infinite range. The lens is moved for focusing in such an amount as described above by rotating the focusing cam using a motor or the like.
A relation between a grooved cam for moving a zoom lens and a cam follower in a zoom optical system is described below. Most of such cam mechanisms comprise a cam groove hole formed in a rotatably disposed cam barrel and a cam follower comprising a roller pin inserted into the cam groove hole and planted in a lens holding frame disposed in the cam barrel so as to be movable in the direction of the lens optical axis. Looseness thus occurs between the cam groove hole and the roller pin.
FIGS. 23A to 23C show the relationship. Assuming that a cam groove hole for driving a first lens groove is C1, a cam follower for the first lens group is D1, a clearance produced between the cam groove hole C1 and the cam follower D1 and causing looseness therebetween is .delta.1, a cam groove hole for driving a second lens group C2, a cam follower D2 for the second lens group, and a clearance produced between the cam groove hole 2C and the cam follower D2 and causing looseness therebetween is .sigma.2, when a zooming operation is stopped at a focal distance F1 in the zooming direction of W.fwdarw.T, the cam followers D1 and D2 constantly contact the image surface sides of the cam groove holes C1 and C2 (i.e. the right-hand sides in FIG. 23A), thereby producing the clearances .delta.1 and .delta.2 on the subject (i.e. left hand) sides of the cam groove holes C1 and C2, respectively, as shown in FIG. 23A. Conversely, when a zooming operation is stopped at the focal distance F1 in the zooming direction of T.fwdarw.W, the cam followers D1 and D2 contact the object sides of the cam groove holes C1 and C2, thereby producing the clearances .delta.1 and .delta.2 on the image surface sides of the cam groove holes C1 and C2, respectively, as shown in FIG. 23B.
In this case, if .delta.1=.delta.2, the focusing value fc causes focusing at a position in front of a desired focal point by .delta.1. If .delta.1.noteq..delta.2, assuming that the longitudinal magnification of the first group cam at the focal distance F1 is .alpha.F1, the change of the value fc is calculated by the following equation: EQU (.delta.1-.delta.2).times..alpha.F1+.delta.2
Focusing is made at a position in front of a desired focal point by the changed value.
However, attention must be paid to the fact that the focal distance only slightly changes during the actual zooming operation. Namely, when the value of the focal distance is detected by an encoder, the actual focal distance F1' is deviated from the focal distance value F1 of the encoder by the clearance .sigma.1 or .sigma.2, as shown in FIG. 23C.
This phenomenon is described in further detail below. If the output of the AF sensor is i, when the zooming operation is stopped after the lens is moved from the W stale to the T state, the extension pulse obtained by using the data of the focal distance F1 is STi, as shown in FIG. 22. When the zooming operation is stopped after the lens is moved from the T state to the W state, the extension pulse obtained by using the focal distance F1' is STi'. Even if the output value of the zoom encoder is the same value F1, data on zooming in the direction toward the T state is thus different from data on zooming in the direction toward the W state.
In order to eliminate the effect of the above phenomenon, both data on zooming in the direction toward the T state and data on zooming in the direction toward the W state are thus stored for the same zoom encoder value so that the focusing value fc can be corrected.
The focusing value fc can also be corrected by providing two systems of data for correcting error of the lens focusing value fc including data on zooming in the direction toward the T state and the data on zooming in the direction toward the W state, as disclosed in the above U.S. Pat. No. 4,914,464.